I have read that corals can change color depending on the tank lighting. I know that I have some green mushrooms in my tank that looked more brown than green in the store, but when I got them used to my tank they turned a brighter green. I read that higher lighting decreased the amount of photosynthetic tissue which causes the color to fade a little, and a little lower lighting can cause the corals to increase their photosynthetic tissue which will cause them to look a little more attractive. You might try moving them to a lower spot in your aquarium so that they don't get quite as much light.